Ionic liquid functionalized reduced graphite oxide / TiO2 nanocomposite for conversion of CO2 to CH4

ABSTRACT

An ionic liquid functionalized reduced graphite oxide (IL-RGO)/TiO 2  nanocomposite was synthesized and used to reduce CO 2  to a hydrocarbon in the presence of H 2 O vapor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. provisional patentapplication No. 61/724,578 entitled “Ionic Liquid Functionalized ReducedGraphite Oxide/TiO₂ Nanocomposite for Conversion of CO₂ to CH₄” filed onNov. 9, 2012, the contents of which are incorporated by reference as ifset forth in its entirety herein for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The invention was made with government support under 1253443 awarded bythe National Science Foundation. The United States government hascertain rights in the invention.

FIELD OF THE INVENTION

The invention relates to a photocatalytic nanocomposite, a relatedmethod of making the photocatalytic nanocomposite, and a method of usingthe photocatalytic nanocomposite to covert carbon dioxide into methane.

BACKGROUND OF THE INVENTION

Photocatalytic materials are drawing significant attention because oftheir potential for solving environmental and energy problems. Amongthese problems are finding ways to address the contributions of CO₂ toglobal warming while still permitting increased energy consumption, asenergy plays a critical role in the quality of life improvement andeconomic prosperity.

One potential avenue for reduction of greenhouse gases such as CO₂ hasbeen CO₂ photoreduction using a photocatalyst in which CO₂ is reduced tovarious less harmful products over the photocatalyst that is activatedby UV radiation. To date, titanium dioxide (TiO₂) is one of the moststudied photocatalysts because it has shown the most efficientphotocatalytic activity, highest stability, low cost, as well as lowtoxicity. In the photocatalytic reaction, electrons and holes areproduced from TiO₂ under UV irradiation. The electrons and holessubsequently interact with reactants (including CO₂) to form theproducts.

However, CO₂ photoreduction has only been performed using titaniumdioxide (TiO₂) with limited or qualified success. One of the problems inusing unmodified TiO₂ as a photocatalyst is that electron and holerecombination leads to low photoconversion efficiency. Although variousmodifications to the catalytic structure have been attempted, none ofthe modifications have created a commercially and industrially viablestructure for CO₂ photoreduction.

Hence, a continued need exists for a photocatalyst that improves thekinetics of a photoreduction reaction of CO₂.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a photocatalytic nanocompositeis provided that includes a reduced graphite oxide, a photocatalyticmetal oxide in nanoparticle form, wherein the photocatalytic metal oxidein nanoparticle form is dispersed in the reduced graphite oxide. Thephotocatalytic metal oxide may be TiO₂ and may be in a mixed phase formincluding rutile and anatase. The reduced graphite oxide may be apowder.

The photocatalytic nanocomposite may further include an ionic moietyattached to the reduced graphite oxide. The ionic moiety may compriseR₁R₂R₃ in which R₁ is NH, R₂ is alkylene, and R₃ is a cationic group. Insome forms, R₂ may be C₁ to C₅ alkylene, and R₃ may be an imidazole ringprotonated or substituted at a nitrogen atom. In one more specific form,R₂ may be propylene and R₃ may be an alkyl-substituted imidazole ring.

In some instances, the reduced graphite oxide may be an ionic liquidfunctionalized reduced graphite oxide formed by attaching aNH₂-terminated ionic liquid to the reduced graphite oxide.

According to another aspect of the invention, a method of making aphotocatalytic nanocomposite of this type is provided. The method ofmaking a photocatalytic nanocomposite includes oxidizing graphite toform a reduced graphite oxide and mixing the reduced graphite oxide withTiO₂ nanoparticles to form the photocatalytic nanocomposite. The reducedgraphite oxide may be functionalized with a NH₂-terminated ionic liquidto form an ionic liquid functionalized reduced graphite oxide before thestep of mixing. The NH₂-terminated ionic liquid may an imidazole suchas, for example, a 1-butyl-3-methylimidazolium-based ionic liquid (e.g.,1-butyl-3-methylimidazolium chloride).

According to still another aspect of this invention a method of CO₂photoreduction is provided. The method of CO₂ photoreduction includescontacting reactants of CO₂ and H₂O over a photocatalytic nanocompositeof the type described herein and reacting the CO₂ and H₂O over thephotocatalytic nanocomposite to produce products including CH₄. In someforms, the products may be substantially free of CO gas. In some forms,the CH₄ may have a production rate in excess of 10 μmol/g catalyst-hr orin excess of 250 μmol/g catalyst-hr.

According to one specific example, an ionic liquid functionalizedreduced graphite oxide (IL-RGO)/TiO₂ nanocomposite was synthesized andused to reduce CO₂ to a hydrocarbon in the presence of H₂O vapor. IL-RGOwas synthesized through chemically attaching the ionic liquid,1-butyl-3-methylimidazolium chloride (C₄mimCl), to a graphite oxide (GO)surface and simultaneously reducing GO to RGO in a basic reactionenvironment. As a comparison, reduced graphite oxide (RGO) wassynthesized using the same procedure, but without adding the ionicliquid. The SEM image revealed that IL-RGO/TiO₂ contained a homogeneousdispersion of graphite flakes with TiO₂ nanoparticles. DiffuseReflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was used tostudy the conversion of CO₂ and H₂O vapor over the IL-RGO/TiO₂ catalyst.Under UV-Vis irradiation, two new peaks were detected in the infraredspectrum after just 40 seconds of irradiation. The intensities of thesepeaks continuously increased in subsequent spectra that were taken underlonger irradiation time. The two peaks detected as products werecharacteristic of CH₄. Background experiments that were conductedconfirmed that CH₄ was indeed generated from the reduction of CO₂ overthe IL-RGO/TiO₂ catalyst surface. A CH₄ production rate of 279 μmol/gcatalyst-hr (as measured using DRIFTS) over a 55 minute period wascalculated. These findings suggest the direct, selective formation ofCH₄ in the absence of CO.

These and still other advantages of the invention will be apparent fromthe detailed description and drawings. What follows is merely adescription of some preferred embodiments of the present invention. Toassess the full scope of the invention the claims should be looked to asthese preferred embodiments are not intended to be the only embodimentswithin the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the overall synthesis of ionic liquid functionalizedreduced graphite oxide (IL-RGO).

FIG. 2 shows the Raman spectra of (a) GO and (b) IL-RGO.

FIG. 3 shows XPS spectra for (a) wide scan survey of IL-RGO/TiO₂, (b)high resolution XPS spectrum of N1s, (c) high resolution XPS spectrum ofC1s in GO, and (d) high resolution XPS spectrum of C1s in IL-RGO.

FIG. 4 shows X-ray Diffraction (XRD) peaks of (a) GO, (b) RGO, and (c)IL-RGO, and (d) XRD peaks of IL-RGO/TiO₂ scanning from 2θ=20°˜70°.

FIG. 5 shows SEM images of (a) IL-RGO/TiO₂ and (b) RGO/TiO₂.

FIG. 6 shows the IR spectra of IL-RGO/TiO₂ surface with CO₂ and H₂Ovapor before and after UV-Visible light irradiation. The IR spectrum ofIL-RGO/TiO₂ was used as a background. The IR spectra were offset forclarification.

FIG. 7 shows the IR spectra of IL-RGO/TiO₂ with H₂O but without CO₂before and after 30 minute UV-Visible irradiation.

FIG. 8 shows energy level diagrams for single phase and mixed phaseTiO₂.

FIG. 9 shows and energy level diagram for the one embodiment of theIL-RGO/TiO₂ structure.

FIG. 10 shows the concentration of methane over IL-RGO/TiO₂ andregenerated sample by cleaning the surface using N₂ at differentUV-Visible irradiation times.

DETAILED DESCRIPTION OF THE INVENTION

Carbonaceous nanomaterials have unique properties and the potential tocontrol the structural and electrical properties of photocatalysts. Thepresence of a carbon material such as carbon nanotubes (CNTs) orgraphene might potentially reduce electron and hole recombination in thephotocatalyst via transport of the electrons to the conductive carbonmaterial. By improving the separation of the charges, recombination maybe avoided thereby enhancing the photoconversion efficiency of TiO₂.Indeed, to date some nanocarbon (for example, CNTs or graphene)/TiO₂composites have shown improved photocatalytic activity over TiO₂ invarious applications (for example, the photooxidation of environmentalpollutants).

As compared to cylindrical CNTs, planar graphene may have a smallerelectron transfer barrier. As a result, the electron-hole recombinationmay be less. Graphene is an atomic sheet of sp²-bonded carbon atoms thatare arranged into a honeycomb structure. The high surface area ofgraphene may increase the adsorption of reactants and provide moreactive sites. Nearly 90% enhancement in photocurrent was seen forreduced graphene oxide described below, which serves as an electroncollector and transporter in the graphene-TiO₂ composite. A significantenhancement in the reaction rate for the degradation of methylene bluewas observed using a P25 (Degussa)-graphene composite material incontrast with a bare P25 (Degussa) or a P25 (Degussa)-CNT material withthe same carbon content. In addition, a decrease in charge transferresistance of graphene/P25 (Degussa) sample was observed versus P25(Degussa) alone.

Presently, there are only a few examples regarding the use of TiO₂nanocarbon materials for the conversion of carbon dioxide (CO₂) and H₂Ovapor to fuels. Xia et al. synthesized multi-walled carbon nanotube(MWCNT) supported TIO₂ and investigated its photocatalytic activity inthe reduction of CO₂ with H₂O (Xia, X. H.; Jia, Z. H.; Yu, Y.; Liang,Y.; Wang, Z.; Ma, L. L., Preparation of multi-walled carbon nanotubesupported TiO₂ and its photocatalytic activity in the reduction of CO₂with H₂O. Carbon 2007, 45, 717-721.). Sol-gel and hydrothermal methodswere used to synthesize the MWCNT/TiO₂ composite. Both syntheses methodsled to the formation of C₂H₅OH, HCOOH and CH₄. The total carbonaceousyields of the products were higher than the reported yields in theliterature, which focused on using other materials to modify TiO₂ ratherthan carbon (i.e., Cu doping, Cu—Fe co-doping TiO₂/porous silica, TiO₂nanotube). This result suggests that carbon-containing TiO₂ materialsmay be better than other materials in terms of enhancing CO₂photoconversion efficiency.

It appears there has been only one instance in which an externally mixedgraphene/TiO₂ material was applied to CO₂ photoreduction. Liang, Y. T.;Vijayan, B. K.; Gray, K. A.; Hersam, M. C., Minimizing Graphene DefectsEnhances Titania Nanocomposite-Based Photocatalytic Reduction of CO₂ forImproved Solar Fuel Production. Nano Letters 2011, 11, 2865-2870. Asolvent exfoliated graphene/P25 thin film and a thermally reducedgraphite oxide/P25 thin film were used as the photocatalysts, and CH₄was the major product identified. The CH₄ production rates of exfoliatedgraphene/P25 thin film and a thermally reduced graphite oxide/P25 thinfilm were 8.1 μmol/m²-hr and 1.8 μmol/m²-hr, respectively. The resultsindicate that TiO₂/graphene may be selective towards CH₄ production butprovided low reaction rates.

In this disclosure, a novel synthesis method was used to create animproved graphene type carbon/TiO₂ composite material for CO₂photoreduction, and the product of the CO₂ photocatalytic reaction inthe presence of water vapor was studied. Instead of adding TiO₂ and thegraphene type material separately, a nanocomposite material was formed.Graphene is a single atomic layer of sp² carbon structure. In thedisclosed method and resulting structure, carbon layers were separatedby oxidizing natural graphite to form graphite oxide and subsequentlyfunctionalizing graphite oxide with a NH₂-terminated ionic liquid. Atthe same time graphite oxide was reacted to form reduced graphite oxide(RGO) in a basic reaction environment. The high solubility of the ionicliquid in water makes it possible for an ionic liquid functionalizedreduced graphite oxide (IL-RGO) to mix well with TiO₂ nanoparticles inwater. Moreover, the ionic liquid, which is functionalized to the RGOsurface, may introduce surface charge to the RGO. The charge repulsionmay help to further separate the graphite layers. The high surface areaof the layer-separated graphite material may enhance the adsorption ofthe reactants, i.e. CO₂ and H₂O vapor, thus creating more reactivesites.

In addition, NH₂-ionic liquid cations may significantly enhance theability to attract CO₂ via amine group-CO₂ interactions. Therefore, theamine-functionalized ionic liquid may also enhance the adsorption ofCO₂. Previous experimental results indicate that the C₄mimCl ionicliquid does not interact with alkanes. CH₄ is a potential product of CO₂photoreduction using TiO₂ or modified TiO₂ catalyst. Therefore, theionic liquid may be able to selectively adsorb the reactant, CO₂, butquickly dissociate the potential product, CH₄, thus promoting thephotoreduction of CO₂.

The IL-RGO/TiO₂ nanocomposite was applied to reduce CO₂ in the presenceof H₂O vapor. The products of CO₂ photoreduction over the IL-RGO/TiO₂was compared with the product formation data that were reported in theliterature using pristine TiO₂ (P25) and other modified TiO₂ in order toprovide an initial examination of the selectivity towards differentproducts.

In the examples provided below, the new method was used to synthesize acarbon/semiconductor composite material via attaching ionic liquid tographite oxide surface to obtain the ionic liquid functionalized reducedgraphite oxide (IL-RGO), and mixing it with TiO₂ nanoparticles insolution. The successful synthesis of this material was confirmed byRaman spectra, XRD, XPS and SEM. Comparing RGO without functionalizedionic liquid, the IL-RGO layers were separated and IL-RGO flakes can beclearly seen in the SEM images. In addition, the SEM images showed thatTiO₂ nanoparticles are dispersed with IL-RGO flakes. The photoreductionof CO₂ over IL-RGO/TiO₂ in the presence of H₂O vapor was investigatedusing Diffuse Reflectance Infrared Fourier Transform Spectroscopy(DRIFTS). CH₄ was formed after just 40 seconds of UV-Vis irradiationover the catalyst of IL-RGO/TiO₂. The IR features of CH₄ increased asthe irradiation time increased. However, no product was found for thephotoreduction of commercial P25 under the same experimental conditions.Therefore, the presence of IL-RGO significantly enhances thephotocatalytic activity of P25 due to the enhanced electron-holeseparation. In addition, CH₄ was found to be the only product forIL-RGO/TiO₂.

The regeneration and reuse of IL-RGO/TiO₂ catalyst for CO₂photoreduction was also investigated. The regenerated catalyst canproduce almost identical amount of CH₄ at the same irradiation times. ACH₄ production rate of 279 μmol/g catalyst-hr over a 55 minute periodwas calculated.

The following examples are presented for illustrative purposes only, andare not intended to limit the scope of the present invention in any way.

EXAMPLES Synthesis of the 1 L-RGO/TiO₂ Composite

Synthesis of the ionic liquid reduced graphite oxide/TiO₂ (IL-RGO/TiO₂)composite involved three basic steps: synthesis of graphite oxide,functionalization and reduction of the graphite oxide in order to makethe material more conductive, and addition of the TiO₂ to allow forphotoactivity.

Synthesis of Graphite Oxide

Graphite Oxide (GO) was synthesized from natural graphite powder (325mesh, Alfa Aesar) by the method of Hummers and Offeman (Hummers, W. S.;Offeman, R. E., Preparation of Graphitic Oxide. Journal of the AmericanChemical Society 1958, 80, 1339-1339). Prior to synthesis using Hummersand Offeman's method, the graphite powder was pre-oxidized. Otherwise,incompletely oxidized graphene-core/GO-shell particles were observed inthe final product. The pre-oxidation procedure followed the method ofKovtyukhova et al. (Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.;Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D.,Layer-by-layer assembly of ultrathin composite films from micron-sizedgraphite oxide sheets and polycations. Chemistry of Materials 1999, 11,771-778.) Briefly, the pre-oxidation process used concentrated H₂SO₄,K₂S₂O₈ and P₂O₅ to oxidize the graphite powder. The resultant productwas subsequently thermally isolated and allowed to cool to roomtemperature. After cooling, the product was diluted and washed withdistilled water until the water's pH became neutral. The product wasdried in air at ambient temperature overnight and subjected to oxidationby Hummers and Offeman's method. The pre-oxidized graphite powder wasfurther oxidized using 0° C. concentrated H₂SO₄ and KMnO₄. The reactionwas terminated by the addition of a large amount of distilled water and30% H₂O₂ solution. The mixture was centrifuged with 1:10 HCl solution toremove metal ions. Also, additional distilled water washing was done tothe mixture until a neutral pH was achieved. The mixture was dark brownin color. The GO was added to distilled water and sonicated for 15minutes to separate the GO layers. Finally, the GO sample was obtainedby centrifugation of the GO solution at 5000 rpm for 30 minutes.

It is observed that this oxidation step may help to prevent the graphitefrom aggregating during mixing, as might occur when using naturalgraphite flakes. This is evidenced by SEM images in the results below inwhich the flakes remain separated.

Ionic Liquid Functionalized Reduced Graphite Oxide (IL-RGO)

The overall reaction for the synthesis of IL-RGO is presented in FIG. 1.A NH₂-terminated ionic liquid of 1-butyl-3-methylimidazolium chloride(NH₂—C₄mimCl) was synthesized using a process that has been reportedpreviously for the synthesis of NH₂-terminated1-butyl-3-methylimidazolium bromide (NH₂—C₄mimBr). See e.g.: Yang, H.F.; Shan, C. S.; Li, F. H.; Han, D. X.; Zhang, Q. X.; Niu, L., Covalentfunctionalization of polydisperse chemically-converted graphene sheetswith amine-terminated ionic liquid. Chemical Communications 2009,3880-3882; and Zhang, Y. J.; Shen, Y. F.; Yuan, J. H.; Han, D. X.; Wang,Z. J.; Zhang, Q. X.; Niu, L., Design and synthesis of multifunctionalmaterials based on an ionic-liquid backbone. AngewandteChemie-International Edition 2006, 45, 5867-5870.

First, 3-chloropropylamine hydrochloride (Sigma Aldrich, 98%) and1-methylimidazole (Sigma Aldrich, 99%) were added to ethanol (SigmaAldrich, ≧99.5%). The mixture was then refluxed under nitrogen for 24hours. The resulting turbid mixture was purified by re-crystallizationfrom ethanol and ethyl acetate as anti-solvent. Finally, the resultingionic liquid was dried under N₂ at 60° C. overnight.

The C₄mimCl functionalized RGO synthesis is based on an epoxidering-opening reaction between GO and NH₂—C₄mimCl. NH₂—C₄mimCl was addedinto a GO dispersed solution. The salt effect of the GO sheet occurreddue to the presence of the ionic liquid. The epoxide ring-openingreaction can be catalyzed by a base. Therefore, KOH was added into theturbid mixture solution. The solution was subjected to ultrasonicationfor 30 minutes. Lastly, the homogeneous solution was vigorously stirredat 80° C. for 24 hours. The resulting solution was washed using ethanoland distilled water several times until the pH was neutral. Theresulting solution was subjected to the IL-RGO/TiO₂ nanocompositematerial synthesis process. The IL-RGO solution was dried at roomtemperature for IL-RGO material characterization. For comparison, RGOwas synthesized using the same procedure, but without adding theNH₂—C₄mimCl.

IL-RGO/TiO₂ Nanocomposite Synthesis

TiO₂ nanoparticles (Evonik P25) were mixed with distilled water and1-Butyl-3-methylimidazolium tetrafluoroborate (C₄mimBF₄, Sigma Aldrich,98%) (H₂O:C₄mimBF₄=9:1 by volume) to make a TiO₂ suspension. Beforemaking the IL-RGO/TiO₂ composite, the IL-RGO solution was ultrasonicatedfor 30 minutes and stirred for 1 hour. Later, the IL-RGO solution wasadded to the TiO₂ suspension and vigorously stirred for one hour. Themass percentage of IL-RGO was 3.5% of the composite material. TheIL-RGO/TiO₂ mixture was washed until the pH became neutral. Then themixture was ultrasonicated for 30 minutes and dried at 80° C. overnight.Eventually, the sample was ground to powder for use in the CO₂conversion experiments. For comparison, the RGO/TiO₂ was synthesizedusing the same procedure.

Material Characterization

Raman spectra of GO and IL-RGO were collected using a custom built Ramanspectrometer in a 180° geometry. The sample was excited using a 100 mWcompass 532 nm laser. The data were collected using an Acton 300ispectrograph and a back thinned Princeton Instruments liquid nitrogencooled CCD detector. The spectral resolution was 3.5 cm⁻¹. X-rayDiffraction (XRD) data were collected with a high resolution X-raydiffractometer (PANALYTICAL X'PERT PRO) using Cu-Kα radiations and anX'celerator detector. Scanning electron microscopy (SEM) was performedusing an XL30 ESEM-FEG. X-ray photoelectron spectroscopy (XPS) wasperformed using a VG ESCALAB 220i-XL aluminum-Kα (1486.6 eV) X-raysource.

The Raman spectra of GO and IL-RGO are shown in FIG. 2. In the Ramanspectrum of GO in FIG. 2a , the G band at 1580 cm⁻¹ is related to thein-plane vibration of the sp² bonded carbon atoms. The D band at 1339cm⁻¹ is associated with the vibration of sp³ bonded carbon atoms, whichcorresponds to the disordered structure of the GO. The D/G bandintensity ratio of GO is 1.03. In the Raman spectrum of IL-RGO in FIG.2b , the D/G band intensity ratio is 0.94. A decrease of the D/G bandintensity ratio of IL-RGO suggests that part of the disorder structureswere restored to in-plane sp² structures. The restoration of sp² carbonstructure indicates an increase in the conductivity of the material.Thus, IL-RGO is likely to transfer the electrons produced from TiO₂under UV irradiation.

X-ray spectroscopy (XPS) was used to characterize the chemicalcomposition of the material. The XPS spectra are presented in FIG. 3.The wide scan survey in FIG. 3a shows that all the expected elements,Ti, O, C and N, are present in the IL-RGO/TIO₂ sample. The highresolution XPS spectra of the IL-RGO/TiO₂ sample were examined for thepresence of the anion of the functionalized ionic liquid, Cl⁻, or theelements from the solvents used in the synthesis process. No peakassociated with Cl⁻ or any element from the solvents used in thesynthesis was found. Therefore, Cl⁻ and solvents used were completelywashed out of the sample. The high resolution XPS spectrum of N1s inFIG. 3b shows that the N1s band appears at 401.7 eV, with a lowerbinding energy shoulder at 399.8 eV. This XPS feature of N1s is shown inboth the IL-RGO/TiO₂ sample and the IL-RGO without adding TiO₂. Thisconfirms the presence of the IL-NH₂ unit in IL-RGO. In addition, thesmall peak of C—N at 286.3 eV in the high resolution XPS spectrum of C1sin the IL-RGO sample in FIG. 3d further confirms that the NH₂ terminatedionic liquid was present in the sample.

The atomic concentration ratio of carbon to oxygen (C/O) determinedusing the XPS data of GO was 0.8, while the C/O ratio for IL-RGOdetermined using the XPS data was 1.4. The atomic concentration of theC—N peak shown in FIG. 3d contributes to 2.6% of the total carbon fromIL-RGO. According to the molecular structure of C₄mimCl, the maximumpercentage of carbon content that could be introduced to IL-RGO bysimply attaching the ionic liquid is 6.0%. If it is assumed that theatomic concentration of oxygen does not change in the process of GOconversion to IL-RGO, the maximum C/O ratio should be 0.9. However, theactual C/O ratio of IL-RGO determined using XPS data is 1.4, suggestingthat a large number of oxygen groups disappeared in the synthesis of theIL-RGO sample. Hence, GO was significantly reduced.

High resolution XPS spectra of C1s in GO and IL-RGO are shown in FIGS.3c and 3d , respectively. Carbon has multiple binding configurations,including graphite C═C, C═O, C—OH, and C in the epoxide/ether. Comparingthe peaks for different binding configurations, the atomic concentrationof the graphite peak (C═C) in GO is 18.6% of carbon in all bindingconfigurations. The maximum atomic concentration of the graphite peakcontributed from the attached ionic liquid is 2.6%. However, the atomicconcentration of graphite peak in IL-RGO accounts for 50.4% of that ofthe carbon in all binding configurations. This confirms that partial sp²graphite structures were restored. In addition, when the IL-RGO wassynthesized from GO, the color of the sample changed from dark-brown todark-gray. The change in color also strongly suggests that GO wasreduced.

Turning now to FIG. 4, the X-ray diffraction (XRD) data show that thediffraction peak of GO appears at 2θ=12.2° in FIG. 4a . This correspondsto an average interlayer space of 0.72 nm. The XRD peak for RGO withoutfunctionalized IL appears at 2θ=12.7° in FIG. 4b , corresponding to theaverage interlayer spacing of 0.70 nm, whereas IL-RGO has a weak andbroad diffraction peak at 2θ=11° in FIG. 4c . As compared to GO, theslightly reduced interlayer space of RGO is likely due to the decreasednumber of oxygen groups. The calculated interlayer spaces of GO and RGOdemonstrate that the interlayer spaces are similar within the GOstructure and RGO structure. Different from the sharp XRD peaks of GO inFIG. 4a and RGO in FIG. 4b , the broad X-ray diffraction peak of theIL-RGO sample in FIG. 4c and its low intensity may be because differentinterlayer spaces were obtained after ionic liquid functionalization,thus suggesting that exfoliation of layered IL-RGO was obtained. FIG. 4dshows the XRD peaks of TiO₂. The relatively noisy XRD spectrum is likelydue to the presence of IL-RGO in the sample. Both anatase and rutilephases are present in the IL-RGO/TiO₂ sample. The anatase TiO₂ accountsfor 73% while rutile phase TiO₂ contributes to 27%. The fractionalcontent determined are very similar to the ratio of anatase and rutileof Degussa P25, which is the TiO₂ precursor in this work.

The scanning electron micrographs (SEM) of IL-RGO/TiO₂ and RGO/TiO₂ areshown in FIG. 5. The separated RGO flakes can be clearly seen in theIL-RGO/TiO₂ sample in FIG. 5a . However, without the functionalized ILas in FIG. 5b , the RGO particles are much larger in the RGO/TiO₂ sampleand aggregate together. A few TiO₂ nanoparticles exist above the RGOaggregates, but the majority of the TiO₂ nanoparticles are covered bythe RGO. Due to the thickness of the multi-layer RGO, the TiO₂nanoparticles below the RGO cannot be clearly seen in the SEM image.When the IL-RGO/TiO₂ solution and the RGO-TiO₂ solution werecentrifuged, the IL-RGO and TiO₂ were well mixed while the RGO and TiO₂were separated. The SEM images reveal that better separation of thegraphite layers can be obtained with the IL-RGO material. The presenceof the functionalized ionic liquid enhances the solubility offunctionalized reduced graphite oxide in water. Thus, a well-mixedIL-RGO and TiO₂ could be obtained in solution.

Photocatalytic Reduction of CO₂ Over IL-RGO/TiO₂ and Bare P25 with H₂OVapor

The experiments on the photoreduction of CO₂ were carried out using aNicolet 6700 Fourier transform infrared (FTIR) spectrometer equippedwith a Praying Mantis diffuse reflectance accessory (Harrick Sci. Corp.,Model DRP-M-07) and a 316 stainless steel high temperature reactionchamber (Harrick Sci. Corp., Model HVC-DRP-4), a mercury cadmiumtelluride (MCT) detector and a KBr beam splitter. The chamber dome hastwo KBr windows and a quartz window. The quartz window was used forvisual observation while the KBr windows were used to permit entry andexit of the infrared beam.

The IL-RGO/TiO₂ or pristine Degussa P25 powders were placed in thesample compartment of the reaction chamber, and the dome was mounted andsealed with an O-ring. The reaction chamber has an inlet for introducinggas to the sample and an outlet for gas exhaust. The chamber was purgedwith N₂ before introducing the CO₂ and H₂O vapor reactant gases. The H₂Ovapor was obtained by flowing N₂ through an impinger containingdistilled water. CO₂ mixed with humidified N₂ was introduced to thereaction chamber. Mass flow controllers (Omega Engineering, Inc.) wereused to control the flow of CO₂ and N₂. The inlet mixing ratio of CO₂ is10% by volume. An IR spectrum was obtained after the CO₂/humidified N₂mixture flowed over the photocatalyst, in order to check that CO₂ andH₂O vapor were absorbed to the surface of the catalyst. The inlet andoutlet of the chamber were sealed, and subsequent spectra were taken ina batch mode of operation.

A series of background experiments were conducted in order tocharacterize the system and to ensure the absence of product formation,even in the presence of the catalytic surface. The sample with CO₂ andhumidified N₂ was kept in the dark for 30 minutes. IR spectra wereobtained during the 30 minutes dark period to examine whether productsformed in the absence of light. In addition, a background experimentwith the catalyst and humidified N₂ but without CO₂ was performed underUV-Visible light irradiation in order to determine whether there wasproduct formation in the absence of CO₂. After performing the backgroundexperiments, the catalyst (IL-RGO/TiO₂ or P25) was placed in the chamberwith CO₂ and humidified N₂. UV-Visible light, produced from adeuterium-halogen light source (Ocean Optics DH-2000-BAL,wavelength=210-1500 nm), was used to activate the catalyst. An opticalfiber cable was used to introduce the light to the sample surfacethrough the quartz window of the chamber. Several IR spectra atdifferent irradiation times were acquired over a total irradiation timeof 60 minutes. Each spectrum was acquired using 4 cm⁻¹ resolution and 32scans.

To attempt to quantify the amount of product formed, standard samples ofproduct (diluted by N₂) were admitted to the DRIFTS reaction cell togenerate a calibration curve. The gas was allowed to equilibrate withthe surface, a spectrum was obtained, and the surface was subsequentlypurged with N₂ in between each admission of product. Spectra wereacquired during the N₂ purge to ensure that the product was completelydesorbed from the surface and removed from the chamber. A surfaceadsorbed product calibration curve was generated and used to attempt toquantify the amount of product formed during the CO₂ photoreductionexperiment over IL-RGO/TiO₂.

The CO₂ photoreduction using IL-RGO/TiO₂ catalyst in the presence of H₂Ovapor was performed twice using fresh and regenerated catalyst samples.Initially, fresh IL-RGO/TiO₂ was used for CO₂ photoreduction. After 30minute UV-Visible irradiation, the sample was regenerated by cleaningthe catalyst surface using pure N₂. IR spectra of the catalytic surfacewere obtained to ensure that the reactants (CO₂ and H₂O vapor), and theCH₄ product formed in the previous run were completely dissociated fromthe catalyst surface and removed from the reaction chamber. Then, thereactants with the same concentrations were admitted to the reactionchamber and the photoreduction experiment was performed again.

In the background experiments, no new peaks were observed in the IRspectra of IL-RGO/TiO₂ with CO₂ and humidified N₂ in the dark over 30minutes. When IL-RGO/TiO₂ and humidified N₂ (in the absence of CO₂) wereirradiated for 30 minutes using UV-Vis light, no new peaks were detectedin the IR spectra. Experiments were also performed for bare P25 with CO₂and humidified N₂. Even after 60 minutes of UV-Vis irradiation, no newpeak formation was detected in the IR spectra.

The IL-RGO/TiO₂ composite material was applied to the photoreduction ofCO₂ in the presence of H₂O vapor. IR spectra of the IL-RGO/TiO₂ surfacebefore and after UV-Visible irradiation were obtained and are shown inFIG. 6. After only 40 seconds of irradiation, new IR features started toappear in the spectrum. In going from 40 seconds to 60 minutes ofirradiation, a new peak at 3017 cm⁻¹ continued to grow in intensity. Thenew peak was initially identified by comparison to the literature asbeing characteristic of CH₄. In addition, a standard IR spectrum of pureCH₄ over IL-RGO/TiO₂ was created in-house and compared with theproduct's IR spectrum. A comparison confirmed that CH₄ was indeed theproduct from reduction of the CO₂ in the presence of water vapor. Thebackground experiments of IL-RGO/TiO₂ with H₂O vapor but without CO₂showed that no peak formed after 30 minutes of UV-Vis irradiation asevidenced by FIG. 7, thus confirming that CH₄ was indeed formed from CO₂reduction in the presence of water vapor rather than from other carbonsources (i.e. RGO or the attached ionic liquid).

The major challenge in CO₂ photoreduction is that the recombination ofelectron and hole generated from TiO₂ is very fast. Thus, there is asignificant decrease in the photocatalytic efficiency of TiO₂. P25,which is the mixed phases of TiO₂ with approximately 75% anatase andapproximately 25% rutile, is expected to have better electron and holeseparation than a single phase of TiO₂ due to the different positions ofthe conduction and valence bands of the anatase and rutile phases suchas is illustrated in FIG. 8.

Electron paramagnetic resonance (EPR) studies by Gray and co-workersindicated that photogenerated electrons actually migrated from rutile tolower energy anatase trapping sites, consequently enhancingelectron-hole separation. See, for example: Hurum, D. C.; Agrios, A. G.;Gray, K. A.; Rajh, T.; Thurnauer, M. C., Explaining the enhancedphotocatalytic activity of Degussa P25 mixed-phase TiO₂ using EPR.Journal of Physical Chemistry B 2003, 107, 4545-4549; Hurum, D. C.;Gray, K. A.; Rajh, T.; Thurnauer, M. C., Recombination pathways in theDegussa P25 formulation of TiO₂: Surface versus lattice mechanisms.Journal of Physical Chemistry B 2005, 109, 977-980; and Hurum, D. C.;Agrios, A. G.; Crist, S. E.; Gray, K. A.; Rajh, T.; Thurnauer, M. C.,Probing reaction mechanisms in mixed phase TiO₂ by EPR. Journal ofElectron Spectroscopy and Related Phenomena 2006, 150, 155-163.

However, when P25 was used in the CO₂ photoreduction experiments, noproduct was observed under the DRIFTS experimental conditions.Therefore, the commercial P25 was still not effective enough for thephotoreduction of CO₂ with H₂O vapor. Nevertheless, it was found thatthe presence of IL-RGO significantly enhances the photoactivity of P25which is likely due to the improved electron-hole separation viaelectron transport from the TiO₂ to the IL-RGO.

The production of CO is frequently reported in the literature as themajor product for CO₂ photoreduction studies using TiO₂-basednanoparticles. The IR feature of CO is expected to appear in the2000-2270 cm⁻¹ frequency range. Notably, the lack of CO features in FIG.6 suggests that there is insignificant production of CO, and CH₄ is theonly product of CO₂ photoreduction in the presence of water vapor overIL-RGO/TiO₂.

Additionally, in the literature, there are different proposed mechanismsfor CH₄ formation in CO₂ photoreduction using TiO₂-based photocatalysts.The selectivity of the products, for example CO or CH₄, may be due todifferences in the numbers of electrons that are produced by thecatalyst and separated from the holes. As reported for photochemicalreduction of CO₂, the reaction mechanism for CO production requires fourelectrons, while eight electrons are needed for CH₄ production. Theselective formation of CH₄ that is reported herein suggests that moreelectrons are available from the IL-RGO/TiO₂ material as compared to thecatalysts used in published studies that have CO as the major product ofCO₂ photoreduction. This further confirms that the presence of IL-RGOhelps to separate electron and hole pairs. The selective production ofCH₄ is very valuable in the application of the catalyst in CO₂photoreduction because CO, as a synthesis gas, cannot be used as a fueldirectly whereas CH₄ is an energy-rich fuel.

In FIG. 10, the concentration of CH₄ formed at different UV-Visibleirradiation times by fresh IL-RGO/TiO₂ catalyst and regeneratedIL-RGO/TiO₂ catalyst are shown. The results show that the regeneratedsample can produce an almost identical amount of CH₄ at the sameirradiation times. These preliminary results suggest that theIL-RGO/TiO₂ catalyst can be regenerated and effectively reused in CO₂photoreduction. This result is significant in that it signifies thereal-world application of this material in CO₂ photoreduction. The CH₄production rate of 279 μmol/g catalyst-hr over a 55 minute period wascalculated. The CH₄ production rate is about 20-30 times higher ascompared to the highest CH₄ production rate using other modified TiO₂structure reported in the literature.

Although the present invention has been described in detail withreference to certain embodiments, one skilled in the art will appreciatethat the present invention can be practiced by other than the describedembodiments, which have been presented for purposes of illustration andnot of limitation. Therefore, the scope of the invention should not belimited to the description of the embodiments contained herein. Forexample, it is contemplated that another application for this materialor other materials made by similar processes may exist in Field EffectTransistors.

What is claimed is:
 1. A photocatalytic nanocomposite comprising: areduced graphite oxide; a photocatalytic metal oxide in nanoparticleform; and an ionic moiety attached to the reduced graphite oxide;wherein the photocatalytic metal oxide in nanoparticle form is dispersedin the reduced graphite oxide.
 2. The photocatalytic nanocomposite ofclaim 1, wherein the ionic moiety comprises R₁R₂R₃ and wherein R₁ is NH,R₂ is alkylene, and R₃ is a cationic group.
 3. The photocatalyticnanocomposite of claim 2, wherein R₂ is C₁ to C₅ alkylene, and R₃ is animidazole ring protonated or substituted at a nitrogen atom.
 4. Thephotocatalytic nanocomposite of claim 3, wherein R₂ is propylene, and R₃is an alkyl-substituted imidazole ring.
 5. The photocatalyticnanocomposite of claim 1, wherein the reduced graphite oxide is an ionicliquid functionalized reduced graphite oxide formed by attaching aNH₂-terminated ionic liquid to the reduced graphite oxide.
 6. Thephotocatalytic nanocomposite of claim 1, wherein the photocatalyticmetal oxide is TiO₂.
 7. The photocatalytic nanocomposite of claim 6,wherein the TiO₂ is in the form of rutile and anatase.
 8. Thephotocatalytic nanocomposite of claim 1, wherein the reduced graphiteoxide is a powder.
 9. A method of making a photocatalytic nanocompositecomprising: oxidizing graphite to form a reduced graphite oxide;attaching an ionic moiety to the reduced graphite oxide; and mixing thereduced graphite oxide with a photocatalytic metal oxide in nanoparticleform to form the photocatalytic nanocomposite.
 10. The method of claim9, further comprising the step of functionalizing the reduced graphiteoxide with a NH₂-terminated ionic liquid to form an ionic liquidfunctionalized reduced graphite oxide before the step of mixing.
 11. Themethod of claim 10, wherein the NH₂-terminated ionic liquid is animidazole.
 12. The method of claim 10, wherein the NH₂-terminated ionicliquid is a 1-butyl-3-methylimidazolium-based ionic liquid.
 13. Themethod of claim 10, wherein the NH₂-terminated ionic liquid is a1-butyl-3-methylimidazolium chloride.
 14. A method of CO₂ photoreductioncomprising: contacting reactants of CO₂ and H₂O over a photocatalyticnanocomposite according to claim 1; reacting the CO₂ and H₂O over thephotocatalytic nanocomposite to produce products including CH₄.
 15. Themethod of claim 14, wherein the products are substantially free of COgas.
 16. The method of claim 14, wherein the CH₄ has a production ratein excess of 10 μmol/g catalyst-hr.
 17. The method of claim 14, whereinthe CH₄ has a production rate in excess of 250 μmol/g catalyst-hr. 18.The method of claim 9, wherein the photocatalytic metal oxide is TiO₂.